Electrochemical sensor having a mediator compound

ABSTRACT

An electrochemical sensor, especially for gases, is provided having a mediator compound based on transition metal salts of polybasic acids and/or transition metal salts of polyhydroxycarboxylic acids. The electrochemical sensor also contains a DLC, BDD or a precious metal thin-layer measuring electrode ( 3 ). The electrochemical sensor may be used for determining SO 2  and H 2 S.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 of German Patent Application DE 10 2006 014 714.6 filed Mar. 30, 2006, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention pertains to an electrochemical sensor, especially for gases, having a mediator compound based on transition metal salts of polybasic acids and/or transition metal salts of polyhydroxycarboxylic acids and to the use thereof.

BACKGROUND OF THE INVENTION

Electrochemical measuring cells are widely used in the analysis of substances, where potentiometry, voltammetry/polarography, coulometry and conductometry can be mentioned as the most important principles of measurement. The use of electrochemical measuring cells for the analysis of gases has also been known for a long time. However, efforts are made to develop new, more sensitive and more reliable sensors, especially when toxic gases in the parts per billion (ppb) range are to be detected and optionally even quantitatively determined.

Such electrochemical gas sensors are required to meet the following requirements for highly sensitive analysis:

-   -   Low residual current I₀;     -   no or at most only very weak effect of variations on the         humidity and/or temperature of the air on the residual current         I₀;     -   low cross sensitivity to interfering gases;     -   low double layer capacity of the measuring electrode, especially         in relation to dynamic measurement methods; and     -   high long-term stability.

The properties of an electrochemical gas sensor are decisively determined by the material, the morphology and the layer thickness of the measuring electrode.

Platinum, gold or graphite are mentioned as measuring electrode materials, e.g., in U.S. Pat. No. 3,795,589. Very many gases can be reacted directly, i.e., without a mediator, on the catalytically highly active precious metals platinum and gold. Therefore, it is frequently impossible to reach the desired selectivity. The long-term stability of the catalytically less active graphite electrodes is slow and these show, depending on the electrode potential, high cross sensitivity to NO and NO₂.

DE 199 39 011 C1 discloses a sensor, whose measuring electrode consists of diamond-like carbon (DLC). DE 101 44 862 A1 pertains to a measuring electrode made of boron-doped diamond (BDD). These electrode materials possess the properties required above, but they require a mediator, which selectively reacts with the analyte.

The use of ferroin (1,10-phenanthroline iron(II) sulfate) as a mediator is proposed in U.S. Pat. No. 3,795,589. However, to make reaction with SO₂ possible, ferroin must be oxidized in the sensor to oxidation state III. This means long running-in periods and high residual currents (“cross-talk” with the auxiliary electrode), which are to be avoided.

SUMMARY OF THE INVENTION

The object of the present invention is to develop a mediator having long-term stability, which is especially selective for sulfur-containing oxidizable gases, especially SO₂. Besides the requirements mentioned in the introduction, it is, furthermore, the object of the present invention to provide a gas sensor with reduced cross sensitivity to interfering gases, short response time and high sensitivity to the analyte. The analyte gas may be present both mixed with other gases or dissolved in a liquid, especially in water.

The subject of the present invention is an electrochemical gas sensor, which contains a novel mediator based on transition metal salts of polybasic acids and/or transition metal salts of polyhydroxycarboxylic acids, besides electrolytes, which are known per se.

The mediator compounds are specifically compounds that contain, besides at least one acid group, at least one other group selected from among hydroxyl and acid groups. In particular, the mediator compound is a carboxylic acid salt having, besides a carboxylic acid group, at least one hydroxyl group, preferably at least two hydroxyl groups, and/or at least one other carboxylic acid group. Tetraborates, such as sodium tetraborate or lithium tetraborate, are also suitable compounds.

Transition metal salts, especially Cu salts of such mediators, permit the selective determination of SO₂. However, such mediator compounds can also be used to determine the concentrations of other target gases, e.g., H₂S.

It was surprisingly found that the Fe³⁺ salts, such as iron hydrogen phthalate and iron phthalate, are especially suitable among the mediator compounds according to the present invention for the determination of H₂S. Formation of elemental sulfur was not observed. Contrary to commercially available sensors, such sensors also do not show cross sensitivity to SO₂.

The mediators according to the present invention possess, furthermore, pH-buffering properties, so that the sensors can be exposed to gas for several hours without loss of sensitivity.

The corresponding Cu²⁺ salts are preferably used to detect or determine SO₂.

The mediators are preferably poorly soluble in the liquid gas sensor composition. The use of suspensions or solutions of the mediator with excess solid offers a number of other advantages, such as:

-   -   Constant mediator concentration with variable air humidity;     -   identical equilibrium potentials at the measuring electrode and         the reference electrode if the reference electrode likewise         consists of carbon;     -   filter action of the excess solid; and     -   the sensor can be operated under anaerobic conditions if the         reference electrode likewise consists of carbon or the mediator         determines the potential thereof.

Hygroscopic alkali or alkaline earth metal halides, preferably chlorides, are preferably used as conductive electrolytes in an aqueous solution. It is also possible to use, e.g., ammonium halides in case of the use of organic solvents, e.g., ethylene carbonate and/or propylene carbonate.

Preferred are measuring electrodes from diamond-like carbon (DLC), especially those known from DE 101 44 862 A1, or measuring electrodes from boron-doped diamond (BDD), whose electrode material makes possible an even larger potential window than DLC electrodes and can also be used in case of extreme requirements, e.g., when determining an analyte with extremely high oxidation potential or very low reduction potential. DLC, which can be manufactured in a simple manner and cost-effectively, is sufficient for many analytes.

The disclosure of DE 199 39 011 C1 and corresponding U.S. Pat. No. 6,607,642, in which measuring electrodes made of diamond-like carbon, and of DE 101 44 862 A1 and corresponding U.S. Pat. No. 6,584,827, in which a measuring electrode made of boron-doped diamond (BDD) is disclosed, are also made hereby expressly part of the disclosure of the present invention by reference (hereby incorporated by reference in their entirety) especially concerning the design of the gas sensor and especially concerning the measuring electrode.

It is suspected that the BDD and DLC electrode material in combination with redox-inactive electrolytes always requires an analyte that is capable of effecting an “outer sphere electron transfer” in contact with the electrode. Since only a small number of target gases accomplish such a charge transfer between the electrode and the target gas according to the experience currently available, it is necessary to add a mediator, which mediates a reaction on the measuring electrode, to the electrolyte solution.

The additional presence of a mediator offers the possibility of making available a sensor that is highly selective against the desired analyte gas by selecting suitable mediators.

In measuring electrodes made of DLC, diamond-like carbon is applied in a very thin layer to a gas-permeable membrane. The diamond-like carbon layer may be produced according to a radiofrequency magnetron sputtering method or by means of other coating methods as well. The thickness of the layer of diamond-like carbon is 50 nm to 1,000 nm.

In case of the design as a BDD electrode, the measuring electrode is applied as a thin layer of boron- or nitrogen-doped diamond to a porous substrate, the porous substrate preferably consisting of a nonwoven material consisting of chemically pure quartz. If the measuring electrode is formed on a porous carrier material, a separate gas-permeable membrane before the measuring electrode may be eliminated.

The thickness of the thin layer of doped diamond is 0.5 μm to 5 μm. In case of a layer consisting of boron-doped diamond, the doping consists of 10¹⁹ to 10²¹ boron atoms per cubic centimeter of diamond. For nitrogen, the doping is approximately 10²⁰ nitrogen atoms per cubic centimeter of diamond.

Besides measuring electrodes consisting of DLC, BDD or precious metal (precious metal sputtered electrode), so-called carbon nanotubes (CNT) are also suitable for use as a measuring electrode material.

Carbon nanotubes are cylindrical carbon molecules from the family of the fullerenes, as they appear, for example, from EP 1 591 417 A1.

Measuring electrodes prepared from carbon nanotubes (CNT) have long-term stability, can be integrated in existing sensor constructions in a simple manner, are suitable for many mediators, and can be purchased at low cost. There are only a small number of cross sensitivities caused by the electrode material. This applies especially to multiwall carbon nanotubes (MW CNT). Such measuring electrodes are wetted by the electrolyte solution over their entire surface, as a result of which a large surface is obtained for the electrochemical reaction.

Carbon nanotubes have a structural relationship to the fullerenes, which can be prepared, e.g., by evaporating carbon according to a laser evaporation method. A single-wall carbon nanotube has, for example, a diameter of one nm and a length of about 1,000 nm. Besides single-wall carbon nanotubes, there also are double-wall carbon nanotubes (DW CNT) and structures with a plurality of walls (MW CNT).

Carbon nanotubes are provided, due to their production, with metal atoms, e.g., Fe, Ni, Co, including the oxides thereof, so that such carbon nanotubes possess catalytic activities on measuring electrodes. It proved to be advantageous to remove these metal particles by acid treatment.

However, it is possible to bind catalysts or mediators (e.g., porphyrins or phthalocyanines) specifically to the carbon nanotubes. However, it is generally preferable to add a soluble mediator to the electrolyte.

The carbon nanotubes are advantageously applied to a porous carrier, a nonwoven or a diffusion membrane. The carbon nanotubes are put together by self-aggregation or with a binder. Polytetrafluoroethylene (PTFE) powder is preferably used as the binder.

It is especially advantageous to prepare the carbon nanotubes from a prefabricated film, a so-called “buckypaper.” The measuring electrode can then be punched out directly from the buckypaper. Large numbers can thus be produced at low cost.

The layer thickness of the carbon nanotubes on the measuring electrode depends on the structure of the measuring electrode.

If the carbon nanotubes are in the form of multiwall carbon nanotubes, the layer thickness is between one μm and 1,000 μm and preferably between 50 μm and 150 μm. The layer thickness is between 0.5 μm and 500 μm and preferably between 10 μm and 50 μm in the case of single-wall carbon nanotubes.

The layer thickness also depends on the purity of the material. The layer thickness is rather at the lower end of the range in case of especially pure material.

The layer thickness of precious metal thin-layer electrodes, which are usually prepared according to a sputtering process, is between 100 nm and 500 nm. The catalytic activity of precious metal thin-layer electrodes is much lower than that of the corresponding thick-layer electrodes, but higher than in DLC or BDD electrodes. The preferred layer thickness of precious metal thick-layer electrodes is between 200 μm and 500 μm.

Classical gas diffusion electrodes (thick layer) are less preferred, because they have a high residual current and low selectivities.

The measuring cell contains the measuring electrode and the auxiliary electrode as well as preferably also a protective electrode and a reference electrode. The sample contains the electrolyte solution and the redox mediator in the dissolved form and optionally also as an excess solid. The measuring cell has openings, which are provided with a membrane permeable to the analyte and otherwise close the measuring cell to the outside. The electrochemical cell contains a measuring electrode, protective electrode, reference electrode and the auxiliary electrode, which may be arranged in a coplanar, plane-parallel or radial arrangement in relation to one another and are flat. The gap between the plane-parallel electrodes may be filled with a separator, which is permeable to the liquid medium and spaces the electrodes apart.

The mode of operation of the measuring cell is as follows: When analyte gas is admitted to the membrane, whether the analyte gas is gaseous or dissolved in a medium, the analyte gas diffuses through the membrane into the electrolyte and is oxidized or reduced by the mediator. The transition metal reduced or oxidized in the process is re-oxidized or re-reduced at the measuring electrode.

The most important processes that take place in the area of the measuring electrode shall be briefly explained below on the basis of the example of Cu²⁺ ions as a component of the mediator and of the analyte gas SO₂. The SO₂ diffusing into the measuring cell from the outside is first oxidized by Cu²⁺, into SO₄ ²⁻:

SO₂+2H₂O+2Cu²⁺⇄SO₄ ²⁻+2Cu⁺+4H⁺.

The resulting Cu⁺ ions are re-oxidized at the measuring electrode:

2Cu⁺⇄2Cu²⁺+2e ⁻

The electrolyte-mediator mixture according to the present invention can be prepared as follows: So much CuCl₂ is added to an LiCl solution that a 0.2-1.0-molar and preferably 0.5-molar CuCl₂ will be formed. The sensor has high sensitivity to SO₂ with this mediator. However, it has a cross sensitivity to H₂S and elemental sulfur is formed, which leads to clogging of the membrane during prolonged exposure to the gas.

The resulting chloro complex can then be mixed, e.g., with potassium hydrogen phthalate, sodium tetraborate or trisodium citrate. The resulting concentration should preferably agree with the above CuCl₂ concentration and be especially about 0.5-molar.

A bluish-green precipitate is formed upon the addition of potassium hydrogen phthalate or sodium tetraborate. Copper hydrogen phthalate, copper phthalate and copper tetraborate were described in the literature as dimeric and polymeric compounds. These substances have not yet been used as mediators so far. This also applies to the copper citrate compound, which is likewise available.

Due to the addition of potassium hydrogen phthalate, sodium tetraborate or trisodium citrate, it was possible to markedly reduce the cross sensitivity to H₂S, surprisingly to completely eliminate the formation of elemental sulfur, to markedly increase the sensitivity to SO₂ and to lower the residual currents.

An exemplary embodiment of the present invention is shown in the figure and will be explained in more detail below.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a longitudinal sectional view of a first electrochemical sensor according to the invention; and

FIG. 2 is a longitudinal sectional view of a second electrochemical sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in particular, in the first embodiment of a first electrochemical sensor 1, which is shown in FIG. 1, a measuring electrode 3 is arranged in a sensor housing 2 behind a diffusion membrane 4. A protective electrode 5, a reference electrode 6, a wick 7 and an auxiliary electrode 8 are also arranged in the sensor housing 2. The interior space of the sensor housing 2 is filled with an electrolyte-mediator mixture. The mediator is additionally also present as an excess solid 10. The measuring electrode 3, the protective electrode 5, the reference electrode 6 and the auxiliary electrode 8 are kept at fixed distances from one another by means of liquid-permeable nonwovens 11, 12, 13, 14.

The gas enters through an opening 15 in the sensor housing 2. The first electrochemical sensor 1 is connected to a potentiostat, which is not shown more specifically, in the known manner.

FIG. 2 shows a second electrochemical sensor 20. Unlike in the first electrochemical sensor 1 according to FIG. 1, the second electrochemical sensor 20 does not have reference electrode 6 as shown in FIG. 1 and instead a disk-shaped reference electrode 16 is arranged behind the protective electrode 5. The second electrochemical sensor 20 otherwise has similar features. The interior space of the sensor housing 2 is filled with an electrolyte-mediator mixture. The mediator is additionally also present as an excess solid 10. Identical components are designated by the same reference numbers as in FIG. 1.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. 

1. An electrochemical gas sensor for detecting an analyte, the electrochemical gas sensor comprising: a measuring electrode; an auxiliary electrode; an electrolyte solution containing the mediator compound, said mediator compound comprising a transition metal salt of an acid compound, said acid compound containing one of at least two acid groups or at least one hydroxyl group and at least one acid group.
 2. An electrochemical gas sensor in accordance with claim 1, wherein said acid compound is a carboxylic acid.
 3. An electrochemical gas sensor in accordance with claim 1, wherein the carboxylic acid is an aromatic carboxylic acid with two or three carboxyl groups.
 4. An electrochemical gas sensor in accordance with claim 1, wherein the carboxylic acid is phthalic acid, isophthalic acid or terephthalic acid.
 5. An electrochemical gas sensor in accordance with at least claim 1, wherein the acid compound is an aliphatic polycarboxylic acid.
 6. An electrochemical gas sensor in accordance with at least claim 1, wherein the acid compound is citric acid.
 7. An electrochemical gas sensor in accordance with claim 1, wherein the acid compound is gluconic acid.
 8. An electrochemical gas sensor in accordance with claim 1, wherein the acid compound is a boric acid.
 9. An electrochemical gas sensor in accordance with claim 1, wherein the electrolyte solution contains alkali or alkaline earth metal salts.
 10. An electrochemical gas sensor in accordance with claim 1, wherein the electrolyte solution contains LiCl.
 11. An electrochemical gas sensor in accordance with at least claim 1, wherein water or organic solvents are used as a solvent.
 12. An electrochemical gas sensor in accordance with at least claim 1, wherein ethylene and/or propylene carbonate are used as a solvent.
 13. An electrochemical gas sensor in accordance with at least claim 1, wherein the measuring electrode is a diamond-like carbon, boron-doped diamond, a precious metal thin-layer electrode or carbon nanotubes.
 14. An electrochemical gas sensor in accordance with claim 9, wherein the thickness of the precious metal thin-layer electrode is 100-500 nm.
 15. An electrochemical gas sensor in accordance with at least claim 1, wherein the transition metal salt is a copper salt or a Cu²⁺ salt.
 16. An electrochemical gas sensor in accordance with claim 11, wherein the Cu²⁺ salt is CuCl₂ and the concentration of CuCl₂ in a 2-10-molar LiCl solution is between 0.2 mol and 1.0 mol.
 17. An electrochemical gas sensor in accordance with claim 11, wherein the Cu²⁺, salt is CuCl₂ and the concentration of CuCl₂ in a 2-10-molar LiCl solution is about 0.5 mol.
 18. An electrochemical gas sensor in accordance with claim 1, wherein the transition metal salt is an iron salt or an Fe³⁺ salt.
 19. An electrochemical gas sensor in accordance with claim 1, further comprising a gas-permeable membrane, said housing having an opening with said gas-permeable membrane wherein the analyte substance enters the area of said measuring electrode via said gas-permeable membrane.
 20. An electrochemical gas sensor in accordance with claim 1, further comprising a reference electrode.
 21. An electrochemical gas sensor in accordance with claim 1, further comprising a protective electrode arranged behind said measuring electrode.
 22. A method of electrochemical gas sensing for detecting an analyte, the method comprising: providing a measuring electrode; providing an auxiliary electrode; providing an electrolyte solution containing the mediator compound, said mediator compound comprising a transition metal salt of an acid compound, said acid compound containing one of at least two acid groups or at least one hydroxyl group and at least one acid group; positioning said measuring electrode and said auxiliary electrode in said electrolyte solution containing the mediator compound to provide an electrochemical gas sensor.
 23. A method according to claim 22, further comprising: using the electrochemical gas sensor for determining SO₂ concentration in a gas wherein said electrolyte is or contains a chloride.
 24. A method according to claim 22, further comprising: using the electrochemical gas sensor for determining H₂S concentration in a gas wherein said electrolyte is or contains a chloride. 